The probe is rumoured to be two years in the making, with party members affiliated with Mr Zhou methodically targeted. If Mr Zhou is expelled from the party and convicted on corruption charges, he would be the highest-ranking party leader to be taken down in decades.

1942: Born in Xi Qiantou village, Jiangsu province, into an eel fisherman's family. Mr Zhou has two younger brothers.

1988-1998: Vice head, then head of the state-owned oil conglomerate China National Petroleum Corp, the largest producer of oil and gas in the country.

2002: Elected to the party's powerful Politburo and elevated to the head of the Ministry of Public Security.

2007: Appointed to the all-powerful, nine-person Politburo Standing Committee, China's top ruling body. He was also promoted to the post of secretary of the law and politics committee. During his tenure as security chief and head of the law and politics committee, Chinese government spending on domestic security exceeded even that for its military/international security.

November 2012: Retired. After the retirement, speculation about his downfall and ensuing investigation begin to swirl following the Bo Xilai scandal. Zhou is thought to be a close ally of Bo, a former rising star in the party who was convicted of corruption.

December 6, 2012: Li Chuncheng, Sichuan's deputy party secretary, became the first senior official to be investigated and targeted in an anti-corruption drive launched by the new leadership. Li was thought to have close ties to Zhou.

September 1, 2013:Jiang Jiemin, head of the State-owned Assets Supervision and Administration Commission, probed for "serious disciplinary violations". Mr Jiang's and Mr Zhou's career paths overlapped, and Mr Jiang's purge is thought to have diminished Mr Zhou's influence.

December 20, 2013: Li Dongsheng, vice minister of public security, under investigation over "serious law and discipline violations". He is a close associate of Zhou.